Synthesis and characterization of nanoalloys and non-precious metal catalysts for energy and environmental applications
Barkholtz, Heather Marie
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Two major project areas are reported in this work. First, a new lithium-assisted dissolution-alloying nanoalloy synthesis technique will be introduced. The development of a new nanoalloy synthesis method is desirable to circumvent issues with traditional co-reduction of metal salt methodology. Co-reduction of metal salts can result in unintended core-shell formation due to different metal ion reduction potentials, as well as oxidation of one component resulting in dealloying of the material when it is exposed to air. In order to best address these issues with a new technique, Pd₃Ag, PdPt, and PdZn systems were studied because Pd₃Ag and PdPt tend to form core-shell structures while PdZn is incredibly difficult to synthesize due to the propensity of Zn to oxidize. The metal nanoalloy systems described in this work (Pd₃Ag, PdPt, and PdZn) were prepared by directly dispersing individual bulk metal alloys (Pd₃Ag) or individual bulk metals (Pd, Pt, and Zn) into molten lithium to form an atomic metal dispersion. The lithium melt is then cooled, removed from the glove box, and converted to LiOH, which allows for the metal atoms to self-assemble into nanoalloys. Alloy structure and nanoscale size were confirmed via XRD, TEM, EXAFS, and XANES methods. The catalytic activity of the Pd₃Ag system was investigated towards the hydrogenation of acrolein, and the PdPt system was used for methanol electro-oxidation. Both Pd₃Ag and PdPt nanoalloy systems gave substantial improvements in their respective catalytic applications over a Pd or Pt standard, respectively. The next project focuses on the rational design and preparation of metal-organic framework (MOF)-derived electrocatalyst materials for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). MOF-derived electrocatalysts are non-platinum group metal (non-PGM) materials and are leading candidates for transportation applications due to their low cost, relatively low operating temperature, capacity to achieve high power density, and direct conversion of chemical to electrical energy acting to improve fuel utilization efficiency. Cathodic ORR is typically sluggish compared to the anodic hydrogen oxidation reaction, requiring more catalyst material to achieve comparable performance. Therefore, research into non-PGM catalysts focuses on the cathodic material design and activation. In this part of the dissertation the focus is to tackle the aggressive milestones set by the U.S. Department of Energy for non-PGM catalysts. First, a series of investigations into the effect of different iron additives and carbon additives on the ORR performance and electrode characteristics was carried out using a one-pot all solid-state synthesis technique to prepare ZIF-8 materials. ZIF-8 is a specific type of MOF which has a Zn²⁺ metal center bonded to two 2-methylimidazole ligands, forming a three-dimensional matrix with high surface area. After the ZIF-8 was prepared, it was ball milled, pyrolyzed, acid washed, and thermally activated once again in a NH₃ environment to yield ORR electrocatalysts consisting of an iron and nitrogen-doped carbon (Fe-N-C) morphology. This one-pot all solid-state synthesis method also allows for investigation into the effect of changing 2-methylimidazole to imidazole on the ORR activity. The effect of this change was determined through catalyst morphology and component characterization and well and electrochemical methods. Finally, ZIF-8 synthesized by wet chemistry was used to prepare the same type of Fe-N-C material as described above. A simple method of tailoring the surface structure and porosity is introduced through careful control of the time spent in NH₃ environment. NH₃ serves to etch the surface of the material as a function of time spent, which results in widening of pores and removal of surface atoms. Careful control and optimization of NH₃ activation conditions allowed for significant improvements in ORR activity in a single fuel cell. Finally, the impact of triple phase boundary penetration on air fed fuel cell data was investigated. Furthermore, it was discovered than when air is used as the oxidant fuel, a break-in period of catalyst improvement exists during the first few polarization curves. It is proposed that scanning to high currents produces water in microporous areas, wetting Nafion ionomer and including those areas in the TPB network.